The neural basis for the generation and control of saccadic eye movements will be studied in squirrel monkeys with physiological and morphological techniques. The axons of long lead burst neurons will be impaled in alert animals, physiologically characterized in relation to gaze, and injected with horseradish peroxidase or biocytin to elucidate neuronal structure-functions correlations. The lectin phasiolis vulgaris, or the anterograde tracer biocytin will be iontophoretically applied to selected sites in the intermediate and deep layers of the superior colliculus to determine whether the sites of mesencephalic reticular formation termination of collicular efferents vary in register with the positions of their somata within the map of oculomotor space represented in the colliculus. Retrograde transport from the reticular formation will also be employed to evaluate these projections. The neural basis for the generation and control of vertical vestibular-induced eye movements, including adaptive plasticity of the vestibulo-ocular reflex (VOR) will also be studied utilizing physiological, morphological, neurochemical, and morphophysiological techniques. The vertical VOR will be visually adapted by employing both minifying and/or magnifying lenses placed in a holder in front of the animals' eyes. Both the normal and adapted VORs will then be utilized as behavioral probes to elucidate the properties of brainstem neurons in vestibulo-ocular pathways. First, we will record extracellularly in alert animals, from Y-group and vestibular nucleus neurons (principally the superior nucleus) in normal monkeys and in those whose vertical VOR has been plastically adapted. We will quantify the responses of these cells to ascertain their roles, if any, in the normal VOR. Then, during VOR adaptation, we will ask if there are changes in neuronal firing that parallel the changes in reflex-induced eye movements. We hypothesize that eye movement plasticity is brought about by modified signals transmitted, pre-formed to the extra ocular motor nuclei. Y-group neurons are favorably situated to carry these signals. They appear to be flocculus target neurons that are only di- or polysynaptically activated by the VIIIth nerve; they project monosynaptically to the IlIrd and IVth nuclei. Evaluating these neurons will test our major hypothesis concerning their role in adaptive plasticity. Discrete chemical lesions will also be employed in the Y-group to temporarily silence it and then the normal and adapted VORs examined. The synaptic connectivity, morphology, and morphophysiology of Y-group neurons will be studied with anatomical tracers and intracellular injection to add information at the cellular is structure in gaze control.